Chronic pain is approaching epidemic proportions in the United States with a recently reported prevalence of one-third in a large population based sample. Pain and depression are closely intertwined, each predicting the severity of the other. Preterm birth, and its concomitant repeated pain exposures, both alters somatosensory processes and contributes to risk for depression in adulthood. We propose to employ a life course modeling study in female and male rats to causally evaluate the relative contribution of two factors shown to render individuals susceptible to persistent pain conditions, early life pain and depression. Repeated early neonatal pain experiences model the painful procedures a very young or preterm neonate might undergo in a neonatal intensive care unit. Upon reaching maturity, animals remain unperturbed or undergo negative mood induction utilizing a chronic and repeated social defeat paradigm, exposures to a dominant resident. Finally, animals undergo hind paw formalin injection, a tonic pain model to assess acute nociceptive behavior in the 60 minutes after injection and measurement of inflammation-induced thermal and mechanical hypersensitivity over subsequent weeks.
The specific aims are to determine: (1) whether repeated early neonatal pain experiences and social defeat increase acute nociceptive behavior following formalin injection;(2) whether repeated early neonatal pain experiences and social defeat increase the severity and persistence of inflammation-induced hypersensitivity over the weeks subsequent to formalin injection;(3) whether social defeat alters baseline (pre- formalin injection) thermal or mechanical sensitivity;(4) the impact of persistent inflammation-induced hypersensitivity on depressive biobehavioral indices;and (5) whether treatment with the tricyclic antidepressant, imipramine, ameliorates the social defeat induced increase in the severity of acute nociceptive behavior. An experimental design over the lifespan addresses Aims 1 - 4, to determine the effects of repeated early neonatal pain experiences and negative mood at maturity on the acute nociceptive behavioral response to hind paw formalin injection and the severity and persistence of the resulting inflammation-induced hyperalgesic state with factors: female vs male;poke vs touch from postnatal day 1 - 8;chronic social defeat vs home cage control;and hind paw formalin vs saline. A second experimental design addresses Aim 5 in mature rats reared under normal conditions with factors, sex, social defeat, and imipramine vs vehicle, with acute nociceptive behavioral response to hind paw formalin injection as the key outcome measure. In addition to the traditional complete factorial design analysis strategy using general linear mixed models, we plan to employ a fractional factorial design in parallel, which has the potential to support the use of fewer animals in complex studies, particularly important in studies involving pain and stress. The significance of the proposed work relates to the ability to causally evaluate the relative contribution of early life pain and depression to the development of persistent pain, and the etiologic validity of the models employed in this life course study.

Public Health Relevance

Nearly 1 in 3 Americans report chronic pain and 1 in 6 suffer major depressive disorder in their lifetime;and pain and depression are closely intertwined, each predicting the severity of the other. Preterm birth, and its concomitant repeated pain exposures, both alters pain processing and contributes to risk for depression in adulthood. The relevance of the proposed study to human health is the use of a lifespan approach to test whether early life pain and depression at maturity increase risk for the development of persistent pain.